When David Karp was 14, he was clearly a bright teenager. Quiet, somewhat reclusive, bored with his classes at the Bronx High School of Science. He spent most of his free time in his bedroom, glued to his computer.

But instead of trying to pry him away from his machine or coaxing him outside to get some fresh air, his mother, Barbara Ackerman, had another solution: she suggested that he drop out of high school to be home-schooled.

“I saw him at school all day and absorbed all night into his computer,” said Ms. Ackerman, reached by phone Monday afternoon. “It became very clear that David needed the space to live his passion. Which was computers. All things computers.”

Clearly.

Now 26 years old, Mr. Karp never finished high school or enrolled in college. Instead, he played a significant role in several technology start-ups before founding Tumblr, the popular blogging service that agreed to be sold to Yahoo for $1.1 billion this week. With an expected $250 million from the deal, Mr. Karp joins a tiny circle of 20-something entrepreneurs, hoodie-wearing characters like Facebook’s Mark Zuckerberg and Foursquare’s Dennis Crowley, who have struck it rich before turning 30.

“When I first met David he was 20 years old and wearing sneakers and jeans,” said Bijan Sabet, a general partner at Spark Capital, who was one of the first people to invest in Tumblr. “But I knew he was one of these rare entrepreneurs that grew up on the Web and who could come up with an idea, build it himself, and then ship it that night.”

Since founding Tumblr six years ago, Mr. Karp has been admired for his programming skills and Web site design acumen but at times has been a polarizing figure in New York tech circles because he so often blogged about his personal life and party-hopping. He has popped up in the New York Post’s Page Six Magazine, and has been a recurring target for the gossip Web site Gawker, where he was labeled a “fameball,” a derogatory term for someone who has an unquenchable desire for fame.

Tall and willowy, with a mop of brown hair and piercing blue eyes, Mr. Karp typically dresses in jeans, a T-shirt and sneakers. He speaks at a rapid clip and, often, for minutes without stopping. Technically, he never graduated from high school, which he cracked in an interview is “hopefully not a condition of Yahoo employment.”

After dropping out and working for a time in small New York tech outfits, Mr. Karp made his way to Tokyo, where he worked for several months for a start-up. He returned to the United States and became the chief technology officer for UrbanBaby, an Internet message board for parents. CNET Networks bought UrbanBaby in 2006, and Mr. Karp took the several hundred thousand dollars he made from the sale to start his own company, called Davidville. One of Davidville’s projects was a simple blogging service called Tumblr.

Mr. Karp’s run at Tumblr has not been without problems. He had trouble hiring in Tumblr’s early days, unsure how to even interview recruits. He often thought large companies were too big for their own good, proclaiming he could manage Tumblr with a team of four.

But Mr. Karp stepped out of the party scene and started dating his current girlfriend, a graduate nursing school student at New York University, four years ago. He also appeared to get more serious about his company as it grew from less than a dozen employees to more than 175 today. “David has grown up in Tumblr,” said Mark Coatney, who oversees Tumblr’s relationship with media companies.

Still, Mr. Karp’s unsure footing led to discussions about his taking a different role at Tumblr, according to two people who worked with the company and who agreed to speak on the condition of anonymity. Because revenue was not growing as fast as they would have liked, investors considered putting Mr. Karp in charge of Tumblr’s product development and finding a more seasoned chief executive.

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David Karp, the founder of Tumblr, on Monday with Marissa Mayer, the chief executive of Yahoo.CreditKarsten Moran for The New York Times

Mr. Karp denied in an interview that there was a plan for him to give up the chief executive’s job. He said that when Tumblr’s chief operating officer left in the middle of last year, filling that job and other critical roles like head of marketing was “top of mind,” but he said there was no plan for him to step aside.

Like many who run so-called social Internet companies, Mr. Karp can teeter on reclusive. In an interview last year at the F.ounders conference in New York, he said he preferred to come to the office early to work alone, avoiding other people.

“Where I feel the most productive and engaged is when I’m buried in code, buried in some project, tweaking some designs,” he said. “I’m certainly introverted.”

Fred Seibert, a television producer who was MTV’s first creative director, first met Mr. Karp in 2000 while he was still in high school. His children attended a private school on the Upper West side of Manhattan called the Calhoun School, where Mr. Karp’s mother was a science teacher.

“My wife and Barbara became very friendly,” Mr. Seibert said. “Over coffee, she was describing how bored David was in high school and my wife said, ‘Fred really likes teenagers, you should send him over.’ And that’s how I met him. He was 14.”

Mr. Karp arrived at Mr. Seibert’s offices on Park Avenue and said he wanted to “learn about engineering and become a good engineer.” At first, Mr. Karp came to the office a few days a week. But then, one day, Mr. Karp announced that he would be coming in every day.

“I asked him if his school schedule changed,” recalled Mr. Seibert. “And David said ‘No, I’ve dropped out!’ ”

Mr. Seibert said it wasn’t long before Mr. Karp became invaluable. He asked him to build the site for his new company, a Web video production outfit called Next New Networks.

“He comes in two weeks later and he hasn’t done it,” Mr. Seibert recalled. “I thought he was being a flaky 19-year-old. But he said, ‘No, no, your idea is just so 2000.’ ”

Mr. Karp pulled out a Sony PlayStation Portable gaming device and told him that soon, Apple would be releasing an iPod with video capabilities. “He said, ‘This is the way people are going to watch video and you really ought to be there.’ ” Next New Networks was one of the first video products on iTunes and was eventually acquired by Google for around $50 million.

“Because of his prescience and timing, we were ahead of the curve,” Mr. Seibert said. Mr. Seibert eventually became an investor in Tumblr and sits on the company’s board.

Mr. Karp, who lives in a $1.6 million one-bedroom loft in Williamsburg, Brooklyn, with his girlfriend and dog, said he is staying put in New York and with Yahoo. He intends to “figure out something” with philanthropy. And one day he might even go to college.

“At least I should be able to afford it,” he said.

Nicole Perlroth and Michael J. de la Merced contributed reporting.

A version of this article appears in print on , Section A, Page 1 of the New York edition with the headline: Before Tumblr, Founder Made Mom Proud. He Quit School.. Order Reprints | Today’s Paper | Subscribe